cifs: don't try to use rdma offload on encrypted connections

The aim of using encryption on a connection is to keep
the data confidential, so we must not use plaintext rdma offload
for that data!

It seems that current windows servers and ksmbd would allow
this, but that's no reason to expose the users data in plaintext!
And servers hopefully reject this in future.

Note modern windows servers support signed or encrypted offload,
see MS-SMB2 2.2.3.1.6 SMB2_RDMA_TRANSFORM_CAPABILITIES, but we don't
support that yet.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: linux-cifs@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>